[ReefWaters]

The only check valves that I know anything about work much like the shut off valves used on waste collectors for beckett skimmers. There is a ball inside a much larger tube. The inlet to the tube is smaller than the ball. When the water is flowing in the "on" direction, the water flows around the ball, when the flow is reversed, the ball is sucked to the inlet tube and stops the flow. I hope this makes sense. If not, let me know.

The reason they fail so often is because there has to be a perfect seal when the flow is reversed. Even a tiny foreign particle, or a small amount of calcium build up, can let water past. Even a drip left un checked can empty a tank over time.

There are two, less than ideal, solutions that I can think of.

Raise the return pipe to the surface of the water in the main tank and angle it back down so you don't have an actual fountain. When the power dies, the siphon will break at the surface. You could also do the same thing but instead of going all the way to the surface, you stop the return pipe at the highest piece of livestock. So if your only having clams and a few small rocks, the pipe could just raise about 6" or so above the sandbed, be hidden a little by rock work, and then as long as you don't have anything alive above the siphon break and your sump can handle the volume, you wont leave anything exposed to the air at all. This is what I would do.

The other, much more complicated, option would be to create an external siphon break. The best way to show this would be a picture but I don't have the time to post one. You would plumb the return from the pump so that the pipe raises above the highest water line and then does a u-turn back down and through the bulkhead in the bottom. Where the u-turn is, you would drill a small hole and attach a small tube or pipe that would drain into the top tank or even the second overflow tank. When the power fails, the siphon would be broken by allowing air into the top of the "u". The water would level out in the tank side of the return at the level of the water in the tank and the water on the pump side would drain down to the sump.

I hope this all made sense and helps a little. I also agree that a little more flow, controlled in a way to keep the surface smooth, would make for better viewing through the sides. My lighting is almost identical to what your planning. I have a 10K 250 with 2x75 actinics and am very interested in seeing the 14K running. I think its an awesome idea and very unique. Good luck getting it dialed in to your liking and keep us all updated.

ReefWaters